1. Field of the Invention
This invention relates generally to large area semiconductor junction devices for use as photocells and in particular to such devices for use as solar cells.
2. Description of the Prior Art
Concern over the continued availability of fossil fuel energy sources has generated interest in the development of other energy sources including solar power which can be used to generate electricity. The devices most often considered for conversion of solar power into electricity are semiconductor devices, commonly called solar cells, which collect light, and generate photocurrent, in proportion to the area of the photosensitive junction which must be large to generate a useful current. The cost of manufacturing such devices depends mainly on the area of the photosensitive junction and is presently too high to permit commercial exploitation of solar cells for other than limited and specialized applications.
Considerable effort has been devoted to finding ways to reduce the cost of semiconductor solar cell devices. Much of this effort has been directed, as in U.S. Pat. No. 3,953,876 issued Apr. 27, 1976, to devices in which the semiconductor material is deposited as a polycrystalline thin film on an inexpensive substrate rather than grown by the costly single crystal techniques used in early solar cells. A different approach that has generated enthusiasm recently is the liquid semiconductor junction solar cells. The active part of these cells is a junction formed at a semiconductor-liquid interface. Because the junction forms spontaneously at the liquid-solid interface, the device promises to be less costly to manufacture as relatively costly epitaxy or diffusion procedures, required for the single crystal or polycrystalline devices mentioned, are not needed to form the junction.
Two obstacles still remain and must be surmounted before such cells can be exploited commercially. First, liquid-semiconductor junctions are often not photochemically stable because photoexcitation produces holes at the semiconductor surface which may react with the redox electrolyte and corrode the semiconductor surface in a manner that degrades the desired characteristics of the semiconductor surface as manifested by decay of the photocurrent from the cell with operating time. An example of such a reaction with a CdS electrode is CdS+2h.sup.+ .fwdarw.S.sup.0 +Cd.sup.2+ leading to the formation of a sulfur layer at the junction interface. One approach to this problem involves the use of a polysulfide-sulfide redox couple solution. Since the corrosion reaction CdS+2h.sup.+ .fwdarw.Cd.sup.++ +S proceeds at a higher electrode potential than the reaction S.sup.= .fwdarw.S+2e, the sulfur-polysulfide couple consumes the holes responsible for the corrosion reaction before the potential for the corrosion reaction is reached.
Second, the cost of single crystal semiconductor electrodes is too high for commercial success. Several approaches have been tried to reduce the cost of the single crystal semiconductor, especially chalcogenide, electrode. One involves the electrolytic codeposition of the electrode materials, e.g., cadmium and selenium, on an inert substrate. Another involves the anodization of a cadmium or bismuth substrate to form a chalcogenide semiconductor.